Optical imaging systems typically include optics that incorporate one or more reflecting and refracting components. When refractive optical elements are used within imaging systems, these components often take the form of lenses with one or two curvature radii crafted within a homogeneous material. Use of refractive optical elements introduces various types of geometric and chromatic aberrations onto an optical image plane.
Chromatic aberration is a type of distortion when a lens is either unable to bring all wavelengths of color to the same focal plane, and/or when wavelengths of color are focused at different positions in the focal plane. Chromatic aberration is caused by lens dispersion, where different colors of light travel at different speeds while passing through a lens, and therefore the lens fails to focus all colors to the same convergence point. This occurs because lenses have different refractive indices for different wavelengths of light. For example, FIG. 1 illustrates a perfect lens with no chromatic aberration, and all wavelengths are focused into a single focal point. However, in reality, the refractive index for each wavelength is different in lenses, which causes two types of chromatic aberration. The first type is longitudinal (or axial) chromatic aberration and is illustrated in FIG. 2. As shown in FIG. 2, longitudinal aberration occurs when different wavelengths of color do not converge at the same point after passing through a lens. The second type of chromatic aberration is lateral (or transverse) chromatic aberration and occurs when different wavelengths of color coming at an angle focus at different positions along the same focal plane, as illustrated in FIG. 3.
Multi-band refractive optical imaging systems are needed for applications such as infrared search and track (IRST) and forward looking infrared (FLIR) systems. For example, in some applications it is desirable that the optical system selectively provide two or more fields of view, such as a wide-angle field of view for general searching of a large area, and a narrow-angle field of view for higher-magnification, more specific analysis of a small portion of the scene of interest. Further, imaging sensors may be used in disparate (different) wavelength ranges, such as the short wave infrared (SWIR, 0.9-1.7 microns), medium wave infrared (MWIR, 3-5 microns) and/or long wave infrared (LWIR, 8-12 microns), such that one field of view is used in one wavelength range, and another field of view is used in another wavelength range. For instance, some applications may require a wide-angle field of view in the LWIR range, and a narrow-angle field of view in the SWIR range. However, there are only a finite number of materials that may be used to design such systems, and determining the best combination of materials can be difficult.